The present invention relates to apparatus and methods for processing semiconductor wafers and, in particular, to a semiconductor wafer etching system in which robotic wafer handling in a vacuum load lock provides rapid, contamination-free loading and unloading of the wafers.
In implementing the dense, complex and contamination-sensitive LSI and VLSI integrated circuit structures, it is desirable, for throughput and particulate control, to utilize a plasma etching technology which employs automatic, batch-type, cassette-to-cassette wafer handling, both for offloading wafer from a cassette onto a wafer support electrode within the processing chamber, and for returning the wafers to a cassette after processing. Throughput and particulate control would also be enhanced by the use of vacuum load lock mechanisms which provide wafer loading and offloading of the wafer support electrode in a vacuum. Load lock mechanisms not only decrease pumping and processing time, but also decrease exposure of the LSI/VLSI structures to contaminants.
Full optimization of throughput and contamination-free wafer handling also requires wafer handling systems which can precisely pick up and release wafers per se without damaging the wafers and without generating particulates from the wafers themselves. Additionally, throughput and cleanliness require wafer handling systems which have the capability to automatically load and unload a wafer support electrode, with a minimum intrusion of particulate-generating mechanisms into the processing chamber. The wafer support electrode typically is polygonal in cross-section and has as many as six or eight wafer-support faces or facets. Such "hexodes" or "octodes" or other multiple-facet electrodes permit a large number of wafers to be processed simultaneously but also impose stringent requirements in precisely positioning and picking up the wafers at a multiplicity of positions on the different faces of the electrode.
Considering first, robotic wafer handling per se, the two robotic-type wafer grippers which are believed to be conceptually the closest existing designs in terms of satisfying the above objectives, were developed in the same time frame as the present invention and, thus, very well may not be prior art. However, these systems are described here because they are the closest known existing designs. One of these wafer chucks is the subject of commonly assigned, co-pending Jacobs et al U.S. patent application, Ser. No. 591,439, filed Mar. 20, 1984, entitled "FINGER CHUCK AND METHOD FOR HANDLING ARTICLES". The Jacobs et al wafer gripper or chuck comprises a plurality of pivoted fingers which cooperatively grip and release a wafer by its edge. Each finger is mounted near the arcuate base of one leg of a U-shaped leaf-spring and extends past the base. The second leg of the spring is mounted to a flat mounting plate. In addition to mounting a wafer gripping finger, the end of the first leg is also mounted to a common base plate and forms a radial configuration with the other spring-mounted wafer gripping fingers. Reciprocal movement of this base plate, either directly by an electromagnetic field or by a solenoid-operated or air-operated plunger, pivots the wafer gripping fingers closed and open about the spring mounting for gripping and releasing a semiconductor wafer.
The second relevant wafer gripper is part of a wafer handling system which is available from Applied Materials, Inc., Santa Clara, Calif. The wafer gripper and associated wafer handling system are disclosed in commonly assigned Flint et al, U.S. Pat. No. 4,457,661, issued July 3, 1984. The Flint et al wafer loading/unloading technique involves removing wafer holding covers or trays from a reactor electrode and mounting them on a generally cylindrical carousel for automatic loading/ unloading of the wafers from the inside of the carousel, that is, from the backside of the trays. In particular, the Flint et al '661 patent covers apparatus for transferring wafers between the trays and a pair of load and unload cassettes which are positioned inside the carousel and trays. The wafers are held on the trays by leaf-spring-mounted clips. The clips are pivoted open by depressor pins mounted on the wafer gripper for inserting and releasing the wafers. The carousel is rotated about its axis to position successive trays for wafer loading and unloading by the gripper assembly. The wafer cassettes are positioned on an elevator assembly and can be indexed both axially (vertically) for alignment with different wafer holding positions on the trays, as well as radially (horizontally) for positioning the two cassettes over a pair of associated wafer transfer blades. The transfer blades transfer the wafers vertically between the cassettes and the associated gripper assembly, which comprises a pair of vacuum chucks. The chucks in turn carry the wafer horizontally between the transfer blades and the wafer holding positions on the trays.
An example of the wafer clips disclosed in the Flint et al '661 patent is shown in FIG. 4A and designated 1 here. Similar clips are disclosed in Dean et al U.S. Pat. No. 4,473,455. The clip depressors used by Flint et al are attached to the vacuum chucks and move with the chucks to engage the clips from the backside of the tray and wafer. The clip depressors (see depressor 2 in FIG. 4A) engage and pivot the clips 1 about their transverse mounting springs 3 in a generally forward and outward direction to open the clip array for gripping or releasing a wafer 4. This clip mounting and construction does not permit engagement from the front side. That is, engagement by the depressor pins 2 from the front side would merely pivot the clips 1 inwardly, closer together. This "closing" of the clip array would prevent loading a wafer 4 onto an empty tray position or, when a wafer is at the tray position, would result in the wafer being gripped even more tightly. In short, the clip construction and operation disclosed in the Flint et al ' 661 and in the Dean et al '455 patent are dedicated to backside loading/unloading of wafer trays, e.g., as described in the Flint et al patent itself, which necessarily involves loading/unloading at a distance from, rather than on, the reactor electrode.
Considering, next, wafer handling systems in general, several different types of approaches have been used for loading/unloading wafer support electrodes. One approach is to load and unload the hexode in the ambient atmosphere, either manually or using automated wafer handling. However, during atmosphere loading and unloading, the wafers and the processing chambers can be contaminated by particulates in the ambient atmosphere and by gases such as water vapor. In addition, this approach decreases throughput because the processing chamber vacuum is broken after each process sequence in order to unload and reload the wafers, and the chamber must then be pumped down to vacuum before starting the next processing sequence.
Another approach is to use a single-wafer processing chamber and load/unload the chamber from a load lock mechanism. This approach educes contamination somewhat, but has the disadvantage of increased wafer handling time and reduced process quality or throughput.
Other approaches include loading multiple wafers on a planar electrode via a load lock mechanism, or onto a horizontally oriented electrode such as a hexode. However, to our knowledge none of the available load lock systems provides automatic, cassette-to-cassette wafer loading and unloading within the load lock onto a wafer-mounting electrode which is vertically oriented in its normal processing orientation. Loading and unloading a vertically oriented cathode is highly desirable because it reduces particulate contamination and allows a higher number of wafers per system floor space. The unavailability of such a system is, no doubt, due to the stringent wafer handling which are required of such a system.
In view of the above state-of-the-art, it is an object of the present invention to provide a robotic wafer handling system which provides automatic, batch-type cassette-to-cassette wafer loading and unloading of a plasma etching/processing chamber using a vacuum load lock.
It is another object to provide a robotic wafer handling system having the characteristics described in the preceding paragraph which loads an unloads a vertical wafer-mounting electrode.
It is yet another object of the present invention to provide a robotic wafer handling system having the characteristics described in the preceding two paragraphs and which meets very stringent particulate and contaminant requirements by virtue of the system design concept of (1) minimizing the generation of particulates and other contaminants by the constituent components or systems and (2) minimizing the transfer of contaminants between the various constituent systems/components.